The present invention relates to a motor drive control. The present invention is suitable for application to an electronic throttle control apparatus for vehicles, specifically for driving a throttle valve disposed in an engine intake pipe by a direct current motor in accordance with a pedal stroke of an accelerator pedal and the like.
U.S. Pat. No. 5,712,550 (Japanese PCT Laid-open Publication JP-A-9-501817) teaches an electronic throttle control apparatus which drives a motor by an H-bridge drive circuit and executes a current limitation while ensuring responsiveness of a valve to be driven. This kind of throttle control apparatus is described with reference to FIGS. 22-26.
As shown in FIG. 22, the electronic throttle control apparatus is for opening and closing a throttle valve 100 by a motor 101. An accelerator sensor 102 detects a pedal stroke of an accelerator 103. A control circuit 104 controls a current supply to the motor 101 through a drive circuit 105 so that the throttle angle is controlled in accordance with the pedal stroke. The drive circuit 105 has four switching devices Q1-Q4 in a H-bridge configuration and connected to the motor 101. A drive logic circuit 106 receives drive command signals A1-A4 from the control circuit (microcomputer and the like) 104 and supplies the current to the motor 101. The current supplied to the motor is monitored by a current detection circuit 107. As shown in FIG. 24, the drive logic circuit 106 has a comparator 108, latches 109 and 110, D flip-flop (DFF) 111, and timers 112, 113 and 114. A high level value Vref(H) and a low level value Vref(L) are provided to be used selectively as a comparison reference for the comparator 108.
As shown in FIG. 25, the motor is supplied with a current of 100% duty ratio to speed up the valve operation during the drive start period (Tst). When the valve is driven to approach a target valve position (Tbr), the motor is supplied with the current in the reverse direction to brake the valve operation. Further, the drive start and braking current is limited to a high current value Ilimit(H). The current limitation is effected by detecting the motor current IM by the current detection circuit 107 and temporarily interrupting the current supply each time the current IM reaches the current limitation value. With this current limitation, the output loss of the drive devices such as power MOSFETs and the like in the Hbridge can be reduced.
On the other hand, a certain limited current is continued to be supplied for a fixed time period even under a lock condition, so that the operation restores from a light lock condition where the valve cannot be controlled to the target position and stays at substantially the same position. This light lock condition may occur from freezing (icing) of water remaining on the valve. Specifically, as shown in FIG. 26, the limit of the current supplied to the motor is switched to a low value Ilimit(L) (&lt;Ilimit(H)), when a motor lock occurs during a period of limiting a motor current IM to the current limitation value Ilimit(H) at the time of motor drive start time (Tst) or braking time (Tbr). The motor lock is detected when the current limitation continues for a predetermined time period (T300).
More specifically, the latch 109 in FIG. 24 stores a history that the current reaches the current limitation value at every predetermined time period To. It is determined that the current limitation is being effected with the current limitation value Ilimit(H) by monitoring the latch 109. The predetermined time period To for this determination is set longer than the switching operation period T1 under the current limitation operation condition.
Thus, the lock condition is determined when the current continues to reach the current limitation value for the predetermined time period, and the current limitation operation is switched to Ilimit(L) thereby to reduce the motor current. As a result, the drive device can be sized smaller for a low cost.
During the current limitation mode by Ilimit(L), the current supply is continued when the motor current IM decreases within the predetermined time period and it is determined that the lock condition has been released. Further, the motor current supply is interrupted when the motor current IM does not decrease during the predetermined time period.
However, as shown in FIG. 26, if the switching time period T2 between the drive start current supply to the braking current supply, it is erroneously determined that the current limitation operation is continuing due to the sum of a limitation period T3 in the drive start current supply period Tst and a limitation period T4 in the braking current supply period Tbr (T3+T4&gt;T300). It is assumed that the history of whether the current has reached the current limitation value within the period longer than the switching period T1 under the current limitation operation is stored to check the lock condition.
The count value which is the sum of the count value T3 in the drive start current supply period Tst and the count value T4 in the braking current supply period Tbr comes to exceed the predetermined value t300, if the current supply condition in which the current does not reach the current limitation value during the predetermined time period in the course from the drive start current supply to the braking current supply is not ensured. As a result, the lock condition is erroneously determined. This erroneous determination causes the current limitation by Ilimit(L) and reduces the output torque at the time of braking current supply. The valve position will undergoes an overshoot as shown in FIG. 26.
According to the conventional apparatus shown in FIG. 25, the current supply is switched to the braking current supply (at timing t200) after the braking current supply at the current value less than the current limitation value is effected for more than the predetermined period To which is for checking the continuation of the current limitation, so that the above erroneous determination may be avoided. That is, the control circuit (microcomputer) 104 produces the drive command signal at the time of switching from the drive start current supply to the braking current supply to ensure the current supply condition of less than the current limitation value for a period (period Tre in FIG. 25) longer than the period To.
Specifically, the control circuit (microcomputer) 104 temporarily produces the command drive signal of 0% duty ratio (current circulation) in the period Tre between the drive start current supply and the braking current supply. The output terminal Q of the D flip-flop 111 in FIG. 24 changes to a L-level and reset the count of the timer 113 once, thereby preventing the erroneous determination.
According to this method, however, the control response characteristics is lowered and the overshoot is caused due to insufficient braking force resulting from the delay of braking current supply after the drive start current supply. Further, the program of the control circuit (microcomputer) 104 which produces the drive command signal becomes complicated.
It is also considered for avoiding other erroneous determination to shorten the drive start current supply time period to slow down the motor operation speed at the time of drive start so that the motor operation speed may be sufficiently reduced even with the braking operation delayed as above. However, the program of the control circuit (microcomputer) 104 becomes very complicated, and the lowering of the original motor operation response characteristics causes lessening of control performance.